1. Field to the Invention
The present invention relates to a liquid crystal display device and a method for fabricating the same, and more particularly, to an in-plane switching (IPS) mode liquid crystal display device and a method for fabricating the same.
2. Discussion of the Related Art
The vast development of information also increases the demands for the development of display devices. Recently, many efforts have been made to study and develop various types of flat display panel devices, such as liquid crystal display (LCD) devices, plasma display panel (PDP) devices, electro luminescent display (ELD) devices, and vacuum fluorescent display (VFD) devices. Some of these types of flat display panel devices have been applied in and incorporated into a range of devices. For instance, LCD devices have been widely used as substitutions for cathode ray tube (CRT) devices in mobile image displays because of their high quality image, light weight, small thickness, compact size and low power consumption. In addition, attempts have been made to incorporate LCD devices into image devices for receiving broadcasting signals, such as televisions and computer monitors. However, LCDs have not been as successfully incorporated into these image devices because the image quality has not been fully satisfactory. In order to implement an LCD device as a general display device, LCD development is dependent on realizing high image quality, high resolution, high luminance, and wide screen, while maintaining light weight, small thickness, compact size and low power consumption.
In general, an LCD device includes a liquid crystal display panel for displaying an image and a driving unit for applying a driving signal to the liquid crystal display panel. The liquid crystal display panel includes first and second glass substrates bonded to each other with a predetermined space therebetween, and a liquid crystal layer formed between the first and second glass substrates. The first glass substrate is commonly referred to as a thin film transistor (TFT) array substrate having a plurality of gate lines arranged along one direction at a predetermined interval from each other, a plurality of data lines arranged in a direction perpendicular to the gate lines at a predetermined interval from each other, a plurality of pixel electrodes formed in a matrix-arrangement within pixel regions defined by the gate and data lines crossing with each other, and a plurality of thin film transistors switched by signals of the gate lines to transfer signals of the data lines to the pixel electrodes. The second substrate is commonly referred to as a color filter substrate having a black matrix layer for cutting off light from a portion except in the pixel regions, an R/G/B color filter layer for producing colored light, and a common electrode for producing an image. However, in an in-plane switching (IPS) mode LCD device, the common electrode is formed on the first glass substrate. In addition, spacers are used to separate the first and second substrates from each other with the predetermined space therebetween, and a sealant is used to bond the first and second substrates to each other.
The liquid crystal layer may be formed in the liquid crystal display panel by employing a liquid crystal injection process. The liquid crystal injection process includes coating a sealant on one substrate to form an injection hole, bonding the substrate to a second substrate, evacuating the space between the bonded two substrates, injecting liquid crystal therein through the injection hole by osmotic pressure phenomenon, and sealing the injection hole with the sealant.
In general, an LCD device uses optical anisotropy and polarization properties of liquid crystal molecules to create images, and the liquid crystal molecules have a natural alignment order. In addition, the alignment order of the liquid crystal molecules can be altered by an electric field, such that the alignment order of the liquid crystal molecules changes as the direction of the electric field being applied to the liquid crystal molecules changes. Accordingly, by applying an electric field to the liquid crystal molecules, light incident on the liquid crystal molecules is then refracted by the changes in the alignment order of the liquid crystal molecules to thereby display images in the LCD device. In particular, in the liquid crystal layer, there are positive liquid crystal molecules with positive(+) dielectric anisotropy, and negative liquid crystal molecules with negative (−) dielectric anisotropy, such that a long axis of the positive liquid crystal molecules is arranged in parallel with a direction of an applied electric field, and a long axis of the negative liquid crystal molecules is arranged in perpendicular to the direction of the applied electric field.
FIG. 1 is a perspective view of an LCD device according to a related art. In FIG. 1, a first substrate 1, functioning as a thin film transistor array substrate, and a second substrate 2, functioning as a color filter array substrate, are bonded to each other with a predetermined space therebetween to form a liquid crystal display panel. A liquid crystal layer 3 is injected between the first and second substrates 1 and 2.
The first substrate 1 has a plurality of gate lines 4 arranged along one direction at a predetermined interval from each other, a plurality of data lines 5 arranged along a direction perpendicular to the gate lines 4 at a predetermined interval from each other, a plurality of pixel electrodes 6 formed in a matrix-arrangement within pixel regions P defined by the gate and data lines crossing with each other, and a plurality of thin film transistors T switched by signals of the gate lines 4 to transfer signals of the data lines 5 to the pixel electrodes 6. In particular, each thin film transistor T has a gate electrode projected from the gate line 4, an active layer formed on a gate insulating film (not shown), a source electrode projected from the data line 5, and a drain electrode opposite to the source electrode. Also, the pixel electrodes 6 are formed of a transparent conductive metal, such as indium-tin-oxide (ITO) having relatively high transmittance. Further, the second substrate 2 has a black matrix layer 7 for cutting off light from a portion except the pixel regions P, an R/G/B color filter layer 8 for producing colored light, and a common electrode 9 for producing an image.
Accordingly, the liquid crystal layer 3 on the pixel electrodes 6 is oriented in response to a signal provided from the thin film transistor T, and an amount of light transmitted through the liquid crystal layer 3 is controlled according to the orientation of the liquid crystal layer 3, thereby creating images. However, such driving of the liquid crystal layer provides images with a poor viewing angle.
FIG. 2 is a cross-sectional view of an IPS mode LCD device according to the related art, and FIGS. 3A and 3B are perspective views of the IPS mode LCD device in FIG. 2. In FIG. 2, an IPS mode LCD device has a first substrate 11 having a pixel electrode 12 and a common electrode 13 formed thereon. The first substrate 11 is bonded with a second substrate 15 with a predetermined space therebetween. A liquid crystal layer 14 operative by an in-plane electric field is formed between the first and second substrates 11 and 15.
In FIG. 3A, the IPS mode LCD device is at an OFF state in which no in-plane electric field is applied to the pixel electrode 12 or the common electrode 13. Thus, no phase change occurs in the liquid crystal layer 14. For example, the liquid crystal layer 14 is twisted to 45° from a horizontal direction of the pixel electrode 12 and the common electrode 13. In FIG. 3B, the IPS mode LCD device is at an ON state in which an in-plane electric field is applied to the pixel electrode 12 and the common electrode 13. Thus, a phase change occurs in the liquid crystal layer 14, such that the liquid crystal layer is twisted by 45° from the orientation in the OFF state shown in FIG. 3A to a direction the same with the pixel electrode 12 and the common electrode 13. The IPS mode LCD device has a wider viewing angle. For example, images created by the IPS mode LCD device can be watched at approx. 70° in up/down/left/right direction when seen from front. However, the IPS mode LCD device has poor transmittance and aperture ratio because the common electrode 13 and the pixel electrode 12 are on the same plane. Also, the IPS mode LCD device has a small misalignment margin of cell gaps, the device is required to have a shorter response time to a driving voltage, and uniform cell gaps.
FIG. 4 is a plan view of an IPS mode LCD device according to the related art, FIG. 5A is an enlarged plan view of ‘A’ of the IPS mode LCD device in FIG. 4, and FIG. 5B is a cross-sectional view of the IPS mode LCD device along I—I′ in FIG. 5A. In FIG. 4, an IPS mode LCD device has an upper substrate 50 and a transparent lower substrate 60. A sealant material 55 is formed on either the upper substrate 50 or the lower substrate 60 for bonding the upper and lower substrates 50 and 60 to each other with a predetermined space therebetween. Also, the IPS mode LCD device includes a liquid crystal injection hole 56 for injecting liquid crystal material between the upper and lower substrates 50 and 60.
In FIG. 5A, the lower substrate 60 includes a plurality of gate lines 61 arranged along one direction at a predetermined interval from each other, a plurality of data lines 64 arranged along in a direction substantially perpendicular to the gate lines 61 at a predetermined interval from each other, and a plurality of pixel electrodes 66b formed within pixel regions defined by the gate and data lines 61 and 64 crossing with each other. In particular, each of the data lines 64 is bent at least once within each pixel region, thereby forming a zigzag pattern. In addition, a thin film transistor is formed in every pixel region. The thin film transistor includes a gate electrode 61a projected from the gate line 61, a gate insulating film 62 (shown in FIG 5B) on an entire surface, an active layer 63 formed on a gate insulating film 62 covering the gate electrode 61a, a source electrode 64a projected from the data line 64, and a drain electrode 66a formed as a unit with the pixel electrode 66b. 
Further, a common line 61b and a plurality of common electrodes 61c are formed on a same layer with the gate lines 61, wherein the common line 61b crosses the pixel region in parallel with the gate line 61, and the common electrodes 61c are formed in a zigzag manner parallel to the data lines 64. The common electrodes 61c are symmetric with respect to the common line 61b. Also, the pixel electrodes 66b are formed in the zigzag manner parallel to and interposed between the common electrodes 61c in the pixel region. The pixel electrodes 66b are also symmetric with respect to the common line 61b. The pixel electrode 66b serves as the drain electrode 66a of the thin film transistor through a contact hole. In addition, the pixel electrode 66b extends to a top portion of a previous gate line to form a storage electrode 66c. 
In FIG. 5B, a passivation layer 65 is formed on an entire surface of the lower substrate 60 above the gate insulating film 62 covering the data lines 64. Also, a black matrix 51, a color filter layer 52, and an overcoat layer 53 are formed on the upper substrate 50. In addition, the liquid crystal layer between each of the common electrodes 61c and an adjacent one of the pixel electrodes 66b is oriented in the same direction by an in-plane electric field applied between the common electrodes 61c and the pixel electrodes 66b, to form one domain. In particular, the IPS mode LCD device have a plurality of domains {circle around (1)}, {circle around (2)}, {circle around (3)}, and {circle around (4)} within one pixel region, thereby providing images with a wide viewing angle. Further, the zigzag patterns of the common electrodes 61c and the pixel electrodes 66b facilitate an alignment direction of the liquid crystal layer within the pixel region in multiple directions, thereby allowing the multiple domains within one pixel region to have different alignment directions from each other.
However, in the IPS mode LCD device, when ion impurities are introduced into and accumulated in active lines (shown in FIG. 4) through the liquid crystal injection hole 56, a relatively high DC charge is generated. This accumulated charge is generated due to a difference in work functions between the metal and the transparent conductive material (ITO), i.e., the common line 61b and common electrode 61c and the pixel electrode 66b. Also, this charge increases the likelihood of trapping ions. Particularly, the last domain {circle around (4)} has a larger DC charge compared to other domains. Unlike the other domains, the last domain {circle around (4)} does not have a signal line on its right side and thus has more ions ⊖ trapped in its aperture region compared to the other domains. These trapped ions ⊖ cause a leakage in the colored light, e.g., a blue light, and the colored light leakage worsens at a low temperature when motions of the ions are slow. In addition, these trapped ions ⊖ cause an effective voltage in the last domain {circle around (4)} to be different from other domains, thereby creating a different in luminance.